1. Field of the Invention
The present invention relates to wireless telecommunications systems and more particularly to the allocation of air interface resources in such a system.
2. Description of Related Art
In a typical cellular radio communications system (wireless telecommunications network), an area is divided geographically into a number of cell sites, each defined by a radio frequency (RF) radiation pattern from a respective base transceiver station (BTS) antenna. The base station antennae in the cells are in turn coupled to a base station controller (BSC), which is then coupled to a telecommunications switch or gateway, such as a mobile switching center (MSC) for instance. The MSC may then be coupled to a telecommunications network such as the PSTN (public switched telephone network) or the Internet.
When a mobile station (MS) (such as a cellular telephone, pager, or appropriately equipped portable computer, for instance) is positioned in a cell, the MS communicates via an RF air interface with the BTS antenna of the cell. Consequently, a communication path is established between the MS and the telecommunications network, via the air interface, the BTS, the BSC and the MSC.
With the explosive growth in demand for wireless communications, the level of call traffic in most cell sites has increased drastically over recent years. To help manage the call traffic, most cells in a Wireless network are usually further divided geographically into a number of sectors, each defined respectively by radiation patterns from directional antenna components of the respective BTS, or by respective BTS antennae. These sectors (which can be visualized ideally as pie pieces) can be referred to as xe2x80x9cphysical sectors,xe2x80x9d since they are physical areas of a cell site. Therefore, at any given instance, an MS in a wireless network will typically be positioned in a given physical sector and will be able to communicate with the telecommunications network via the BTS serving that physical sector.
In a CDMA (code division multiple access) system, each cell site may employ one or more carrier frequencies for communication with the mobile stations that are in its boundaries. The number of carrier frequencies employed by a given cell site may depend on various factors, such as the density of communication traffic expected in the site, for instance. In a congested city area, for example, a given cell site might be designed to employ three or four carrier frequencies, while, in a sparsely populated rural area, a cell site might employ only one or two carrier frequencies.
When a cell site employs more than one carrier frequency, the cell cite may be considered to have a multiple of its number of physical sectors. For instance, if a cell site is divided physically into three sectors by directional antenna elements, and the cell site employs four carrier frequencies, then the cell site may effectively have twelve sectors, three operating at each of the four carrier frequencies. (Variations on this arrangement are, of course, possible as well.) For convenient reference, these sectors may be referred to as xe2x80x9cfrequency-sectorsxe2x80x9d, since each sector could vary from another by its carrier frequency and its physical location.
Each physical sector in a CDMA system is also distinguished from adjacent physical sectors by a PN offset number or key. When a mobile station is present in a given physical sector, communications between the mobile station and the BTS of the cell site are encoded by the physical sector""s PN offset key, regardless of the carrier frequency being used.
In normal operation, when a mobile station is engaged in a call on a given frequency and the mobile station moves into a new physical sector, the call will continue on that same frequency in the new physical sector. Through communication with a base station controller, the mobile station will simply switch to use the PN offset key of the new physical sector in order to complete the handoff from one physical sector to the next.
The present invention stems from a realization that problems can exist in the normal allocation of air resources as described above. For example, in many cases, a new physical sector into which a mobile station roams may not employ the same carrier frequency as the physical sector from which the mobile station came. (This often occurs, for instance, when a mobile station moves from a city area into a rural area where fewer carrier frequencies are provided.) In that scenario, the mobile station most likely drop the call, because communications on the carrier frequency that the mobile station was using will be discontinued.
As a possible solution to this problem, a pilot beacon can be used to emit communications signals into a physical sector on carrier frequencies that would otherwise be unavailable in that physical sector. With this arrangement, when a mobile station moves from a physical sector using a given carrier frequency into another physical sector in which that carrier frequency is not available, signals can still be sent to the mobile station on that carrier frequency in order instruct the mobile station to switch to a carrier frequency that is available in the new physical sector. Unfortunately, however, such pilot beacons are expensive. Therefore, this solution is not desirable.
A problem of a different type can also arise even when a mobile station moves between physical sectors that share the carrier frequency being used by the mobile station. For instance, a mobile station, might move from a first physical sector operating on carrier frequency A to a second physical sector operating on carrier frequency B However, it is possible that carrier frequency A in the second physical sector is exceptionally congested with mobile station traffic at the moment. Consequently, as the mobile station enters the second physical sector and continues to communicate on carrier frequency A, the quality of communications on that carrier in the second physical sector could be quite poor, and the added traffic could result in dropped calls as well. In this instance, it is possible that another carrier frequency B also employed in the second physical sector is not particularly congested at the moment. Therefore, it might be more efficient if the mobile station would change to carrier B as it moves into the second physical sector, thereby avoiding the increase in congestion on carrier A.
As still another example, as a mobile station moves from one cell site to another, the mobile station will typically pass through an area where the two cell sites (and consequently at least two physical sectors) overlap. In that transitional area, a choice must be made as to whether the mobile station should continue to communicate with the first cell site or should instead communicate with the second cell site. In existing systems, as the mobile station moves progressively further away from the BTS of the first cell site, the mobile station will continue to increase its transmission power in order to stay in touch with the BTS of the first cell site. Unfortunately, however, that increase in power could effectively raise the noise level in the first cell site (as higher power signals are transmitted within the cell) and could consequently reduce capacity in the first cell site. It is possible that some other carrier frequency in either the first or second cell site could be far less congested and subject to less interference from high transmission levels of the mobile station. Therefore, it might be more efficient if the mobile station would use that other carrier instead.
An exemplary embodiment of the present invention can help solve the foregoing problems, by seeking to optimize the allocation of air interface resources in a wireless network. In an exemplary embodiment, a network entity uses information about the location of a mobile station, together with information about actual congestion on various carrier frequencies in areas of a wireless network, to determine (i) when communications with a mobile station can be switched to a different carrier frequency, (ii) when communications with the mobile station should be switched to a different carrier frequency, and (iii) to which carrier frequency the communications should be switched.
As a general matter, information about the location of a mobile station may be collected and monitored. The information can be regularly collected by the mobile station, for instance, through use of a GPS (global positioning system) receiver or other technology, and the mobile station can regularly provide the information to a controller in the network. Alternatively, the information may be collected by any of a number of other methods. The controller may monitor this information to determine when a situation exists where the mobile station could be switched to a different carrier frequency. For instance, if the mobile station is in, or is moving into, an area that employs multiple carrier frequencies, the controller may determine that an option exists to switch the mobile station to another frequency.
When the controller determines that an option exists to use another carrier frequency for communications with the mobile station, the controller conducts (or causes to be conducted) an analysis to select what it finds to be an optimal carrier frequency. This analysis may be as simple as determining which carrier frequency in the area at issue is least congested at the moment, or the analysis could be more complex, taking into consideration factors such as power levels, noise levels and the like.
If the optimal carrier frequency is the frequency on which the mobile station is already operating, then the controller need take no further action. Alternatively, if the optimal carrier frequency is a frequency other than that on which the mobile station is currently operating, then the controller may orchestrate a handoff of the mobile station to the optimal frequency.